Recently a new class of semiconductor lasers, designated "quantum cascade" or "QC" lasers, was discovered. See, for instance, U.S. Pat. Nos. 5,457,709, 5,509,025 and 5,570,386, all incorporated herein by reference. See also U.S. patent applications Ser. No. 08/825,286 (filed Mar. 27, 1997) and U.S. Pat. No. 08/744,292 (filed Nov. 6, 1996), both incorporated herein by reference.
A "quantum cascade" or "QC" laser herein is a unipolar semiconductor laser having a multilayer structure that forms an optical waveguide, including a core region of relatively large effective refractive index between confinement regions of relatively small effective refractive index. The core region comprises a multiplicity of nominally identical repeat units, with each repeat unit comprising an active region and a carrier injector region. The active region has a layer structure selected to provide an upper and a lower carrier energy state, and such that a carrier transition from the upper to the lower energy state results in emission of a photon of wavelength .lambda.. The carrier injector region has a layer structure selected to facilitate carrier transport from the lower energy state of the active region of a givcn repeat unit to the upper energy state of the active region of the adjacent (downstream) repeat unit. A carrier thus undergoes successive transitions from an upper to a lower energy state as the carrier moves through the layer structure, resulting in the creation of a multiplicity of photons of wavelength .lambda.. The photon energy (and thus .lambda.) depends on the structural and compositional details of the repeat units, and possibly also on the applied electric field and/or a distributed feedback element.
Although prior art QC lasers can be designed to emit at wavelengths in a wide spectral region, it would be desirable to have available a QC laser that can be designed for emission at wavelengths in the so-called first atmospheric window (approximately 3-5 .mu.m). Such a laser would be important for a variety of applications, e.g., sensing of trace amounts of gases, for instance HCl.
Because QC lasers are based on carrier transitions between energy levels created by quantum confinement, attainable emission is limited on the short wavelength side by the size of the conduction band discontinuity between the two semiconductor materials of the active region. Prior art QC lasers generally are based on In.sub.0.53 Ga.sub.0.47 As/In.sub.0.52 Al.sub.0.48 As, which is lattice matched to InP and has a conduction band discontinuity .DELTA.E.sub.c =520 meV. In this system maximum photon energies of about 250 meV (.about.0.48.DELTA.E.sub.c) were realized at temperatures T.gtoreq.300 K.
To attain good coverage of the 3-5 .mu.m atmospheric window, it would be desirable to have available a QC laser having heterojunctions with larger conduction band discontinuity than are available in prior art QC lasers. This application discloses such a QC laser.